A direct detection system generally consists of a pair of an optical intensity modulator and a photoelectric detector. At the present stage, the direct detection system mainly supports an intensity-modulated OOK (On-Off Keying) signal, a PAM (Pulse Amplitude Modulation) signal, or further complex signals such as a DMT (Discrete Multi-Tone) signal and a CAP (Carrier-less Amplitude Phase) signal. These signal formats are commonly characterized in that: only the intensity information of light is involved without the phase information. Therefore, it is only needed to adopt an optical intensity modulator (such as a direct modulation laser or an electro absorption modulator) on the transmitting side so as to fulfill the conversion from electric signal to light signal. Accordingly, it is only needed to adopt a photoelectric detector to fulfill the photo-electric conversion. The simple structure and ease of implementation led the above system to be the preferred solution for a low-cost optical transmission system.
However, with the continuous upgrading of the internet service, users are proposing higher demands on the performance of the optical transmission network. For example, a higher spectrum efficiency, a longer transmission distance and a low-cost and low-power solution are excepted. Though the traditional direct detection method maintains a cost advantage, it can hardly support a phase modulation format such as QAM (Quadrature Amplitude Modulation) and OFDM (Orthogonal Frequency Division Multiplexing), since the amplitude and phase information of the transmission signal cannot be detected simultaneously. In addition, in cases in which the signal bandwidth is extremely high, the performance of the system suffers from chromatic dispersion, thus it is difficult to implement a long distance transmission without dispersion compensation fiber.